Evolution Heats Up in the Tropics

Time for a pop quiz. Which mammal evolves more rapidly: the ringtail, a raccoonlike animal common in the southwestern United States, or the closely related cacomistle, which lives in tropical forests in southern Mexico and Central America? The answer is the cacomistle, according to a surprising new study indicating that lower elevations and lower latitudes seem to speed up microevolution in mammals.

For years, many scientists would have considered that to be a trick question. Previous studies showed that plants living at low latitudes and elevations did evolve more quickly, most likely because the warmer temperatures boosted their metabolic rate, which in turn increased their mutation rate. But most researchers believed that mammals wouldn't respond in the same way because warm-blooded creatures maintain a fairly constant body temperature no matter the outside temperature, so any connection between climate and metabolism would be moot.

But the pattern of mammal biodiversity--extensive in low elevations and nearer the equator, sparse in high places and nearer the poles--suggested a connection to Len Gillman, an evolutionary ecologist at the Auckland University of Technology in New Zealand. "I thought, 'There's got to be a universal explanation.' "

So Gillman and his colleagues spent the next year and a half analyzing 130 closely related species pairs, such as the ringtail and cacomistle, in which one species lives at a lower latitude or elevation than its sister species. Using GenBank, a public database of DNA sequences, the team compared the sequence of the cytochrome b gene--the most consistently available genetic marker in mammals--in each species and in a common ancestor they both shared. The more mutations in the gene, the faster that species has evolved since it split from the common ancestor.

Gillman and colleagues report today in the Proceedings of the Royal Society B that species living at lower elevations and lower latitudes seem to have evolved on average about 1.5 times more quickly than their sister species. The team has two possible explanations for this pattern. First, animals in colder climates are less active, often resting or hibernating to conserve energy. Some studies suggest a link between activity and annual average metabolic rate, which could mean fewer mutations in languid species.

The second explanation derives from the Red Queen Hypothesis, which posits an evolutionary arms race between predators and prey. Gillman says that if plants in warmer climates evolve more quickly, mammals living there will also have to speed up their evolution to keep pace with their food's rapidly adapting defenses.

Jason Weir, an evolutionary ecologist at the University of Chicago in Illinois whose past work has found no connection between latitude and rate of evolution in birds, says that Gillman's study is interesting but overlooks an important factor: generation time. Because the tropics favor breeding almost all year long, animals living there produce subsequent generations more quickly. Mammals in colder areas, in contrast, breed less often. "A large generation time is often believed to result in slower mutation rates," Weir explains.

Marcel Cardillo, an evolutionary ecologist at the Australian National University in Canberra whose previous work also came to different conclusions than Gillman's, shares Weir's concerns about generation time. But he says that Gillman's methods are sound and the results are "potentially another important piece in the puzzle of why there are more species in the tropics."